6.3.3 Converting Single Characters

The most fundamental of the conversion functions are those dealing with
single characters. Please note that this does not always mean single
bytes. But since there is very often a subset of the multibyte
character set that consists of single byte sequences, there are
functions to help with converting bytes. Frequently, ASCII is a subpart
of the multibyte character set. In such a scenario, each ASCII character
stands for itself, and all other characters have at least a first byte
that is beyond the range 0 to 127.

— Function: wint_t btowc (int c)

The btowc function (“byte to wide character”) converts a valid
single byte character c in the initial shift state into the wide
character equivalent using the conversion rules from the currently
selected locale of the LC_CTYPE category.

If (unsigned char) c is no valid single byte multibyte
character or if c is EOF, the function returns WEOF.

Please note the restriction of c being tested for validity only in
the initial shift state. No mbstate_t object is used from
which the state information is taken, and the function also does not use
any static state.

The btowc function was introduced in Amendment 1 to ISO C90
and is declared in wchar.h.

Despite the limitation that the single byte value always is interpreted
in the initial state this function is actually useful most of the time.
Most characters are either entirely single-byte character sets or they
are extension to ASCII. But then it is possible to write code like this
(not that this specific example is very useful):

Why is it necessary to use such a complicated implementation and not
simply cast '0' + val % 10 to a wide character? The answer is
that there is no guarantee that one can perform this kind of arithmetic
on the character of the character set used for wchar_t
representation. In other situations the bytes are not constant at
compile time and so the compiler cannot do the work. In situations like
this it is necessary btowc.

There also is a function for the conversion in the other direction.

— Function: int wctob (wint_t c)

The wctob function (“wide character to byte”) takes as the
parameter a valid wide character. If the multibyte representation for
this character in the initial state is exactly one byte long, the return
value of this function is this character. Otherwise the return value is
EOF.

wctob was introduced in Amendment 1 to ISO C90 and
is declared in wchar.h.

There are more general functions to convert single character from
multibyte representation to wide characters and vice versa. These
functions pose no limit on the length of the multibyte representation
and they also do not require it to be in the initial state.

The mbrtowc function (“multibyte restartable to wide
character”) converts the next multibyte character in the string pointed
to by s into a wide character and stores it in the wide character
string pointed to by pwc. The conversion is performed according
to the locale currently selected for the LC_CTYPE category. If
the conversion for the character set used in the locale requires a state,
the multibyte string is interpreted in the state represented by the
object pointed to by ps. If ps is a null pointer, a static,
internal state variable used only by the mbrtowc function is
used.

If the next multibyte character corresponds to the NUL wide character,
the return value of the function is 0 and the state object is
afterwards in the initial state. If the next n or fewer bytes
form a correct multibyte character, the return value is the number of
bytes starting from s that form the multibyte character. The
conversion state is updated according to the bytes consumed in the
conversion. In both cases the wide character (either the L'\0'
or the one found in the conversion) is stored in the string pointed to
by pwc if pwc is not null.

If the first n bytes of the multibyte string possibly form a valid
multibyte character but there are more than n bytes needed to
complete it, the return value of the function is (size_t) -2 and
no value is stored. Please note that this can happen even if n
has a value greater than or equal to MB_CUR_MAX since the input
might contain redundant shift sequences.

If the first n bytes of the multibyte string cannot possibly form
a valid multibyte character, no value is stored, the global variable
errno is set to the value EILSEQ, and the function returns
(size_t) -1. The conversion state is afterwards undefined.

mbrtowc was introduced in Amendment 1 to ISO C90 and
is declared in wchar.h.

Use of mbrtowc is straightforward. A function that copies a
multibyte string into a wide character string while at the same time
converting all lowercase characters into uppercase could look like this
(this is not the final version, just an example; it has no error
checking, and sometimes leaks memory):

The use of mbrtowc should be clear. A single wide character is
stored in tmp[0], and the number of consumed bytes is stored
in the variable nbytes. If the conversion is successful, the
uppercase variant of the wide character is stored in the result
array and the pointer to the input string and the number of available
bytes is adjusted.

The only non-obvious thing about mbrtowc might be the way memory
is allocated for the result. The above code uses the fact that there
can never be more wide characters in the converted results than there are
bytes in the multibyte input string. This method yields a pessimistic
guess about the size of the result, and if many wide character strings
have to be constructed this way or if the strings are long, the extra
memory required to be allocated because the input string contains
multibyte characters might be significant. The allocated memory block can
be resized to the correct size before returning it, but a better solution
might be to allocate just the right amount of space for the result right
away. Unfortunately there is no function to compute the length of the wide
character string directly from the multibyte string. There is, however, a
function that does part of the work.

The mbrlen function (“multibyte restartable length”) computes
the number of at most n bytes starting at s, which form the
next valid and complete multibyte character.

If the next multibyte character corresponds to the NUL wide character,
the return value is 0. If the next n bytes form a valid
multibyte character, the number of bytes belonging to this multibyte
character byte sequence is returned.

If the first n bytes possibly form a valid multibyte
character but the character is incomplete, the return value is
(size_t) -2. Otherwise the multibyte character sequence is invalid
and the return value is (size_t) -1.

The multibyte sequence is interpreted in the state represented by the
object pointed to by ps. If ps is a null pointer, a state
object local to mbrlen is used.

mbrlen was introduced in Amendment 1 to ISO C90 and
is declared in wchar.h.

The attentive reader now will note that mbrlen can be implemented
as

mbrtowc (NULL, s, n, ps != NULL ? ps : &internal)

This is true and in fact is mentioned in the official specification.
How can this function be used to determine the length of the wide
character string created from a multibyte character string? It is not
directly usable, but we can define a function mbslen using it:

This function simply calls mbrlen for each multibyte character
in the string and counts the number of function calls. Please note that
we here use MB_LEN_MAX as the size argument in the mbrlen
call. This is acceptable since a) this value is larger then the length of
the longest multibyte character sequence and b) we know that the string
s ends with a NUL byte, which cannot be part of any other multibyte
character sequence but the one representing the NUL wide character.
Therefore, the mbrlen function will never read invalid memory.

Now that this function is available (just to make this clear, this
function is not part of the GNU C library) we can compute the
number of wide character required to store the converted multibyte
character string s using

wcs_bytes = (mbslen (s) + 1) * sizeof (wchar_t);

Please note that the mbslen function is quite inefficient. The
implementation of mbstouwcs with mbslen would have to
perform the conversion of the multibyte character input string twice, and
this conversion might be quite expensive. So it is necessary to think
about the consequences of using the easier but imprecise method before
doing the work twice.

The wcrtomb function (“wide character restartable to
multibyte”) converts a single wide character into a multibyte string
corresponding to that wide character.

If s is a null pointer, the function resets the state stored in
the objects pointed to by ps (or the internal mbstate_t
object) to the initial state. This can also be achieved by a call like
this:

wcrtombs (temp_buf, L'\0', ps)

since, if s is a null pointer, wcrtomb performs as if it
writes into an internal buffer, which is guaranteed to be large enough.

If wc is the NUL wide character, wcrtomb emits, if
necessary, a shift sequence to get the state ps into the initial
state followed by a single NUL byte, which is stored in the string
s.

Otherwise a byte sequence (possibly including shift sequences) is written
into the string s. This only happens if wc is a valid wide
character (i.e., it has a multibyte representation in the character set
selected by locale of the LC_CTYPE category). If wc is no
valid wide character, nothing is stored in the strings s,
errno is set to EILSEQ, the conversion state in ps
is undefined and the return value is (size_t) -1.

If no error occurred the function returns the number of bytes stored in
the string s. This includes all bytes representing shift
sequences.

One word about the interface of the function: there is no parameter
specifying the length of the array s. Instead the function
assumes that there are at least MB_CUR_MAX bytes available since
this is the maximum length of any byte sequence representing a single
character. So the caller has to make sure that there is enough space
available, otherwise buffer overruns can occur.

wcrtomb was introduced in Amendment 1 to ISO C90 and is
declared in wchar.h.

Using wcrtomb is as easy as using mbrtowc. The following
example appends a wide character string to a multibyte character string.
Again, the code is not really useful (or correct), it is simply here to
demonstrate the use and some problems.

First the function has to find the end of the string currently in the
array s. The strchr call does this very efficiently since a
requirement for multibyte character representations is that the NUL byte
is never used except to represent itself (and in this context, the end
of the string).

After initializing the state object the loop is entered where the first
task is to make sure there is enough room in the array s. We
abort if there are not at least MB_CUR_LEN bytes available. This
is not always optimal but we have no other choice. We might have less
than MB_CUR_LEN bytes available but the next multibyte character
might also be only one byte long. At the time the wcrtomb call
returns it is too late to decide whether the buffer was large enough. If
this solution is unsuitable, there is a very slow but more accurate
solution.

Here we perform the conversion that might overflow the buffer so that
we are afterwards in the position to make an exact decision about the
buffer size. Please note the NULL argument for the destination
buffer in the new wcrtomb call; since we are not interested in the
converted text at this point, this is a nice way to express this. The
most unusual thing about this piece of code certainly is the duplication
of the conversion state object, but if a change of the state is necessary
to emit the next multibyte character, we want to have the same shift state
change performed in the real conversion. Therefore, we have to preserve
the initial shift state information.

There are certainly many more and even better solutions to this problem.
This example is only provided for educational purposes.